5:15 PM - 7:15 PM
[SVC34-P20] Petrological contrasts between crystal-rich and crystal-poor silicic magmas: A case study of Kuju and Aso volcanoes, Southwest Japan
Keywords:Silicic magma, Caldera, Petrology, Crystal mush, Magma reservoir
Kuju volcano and Aso volcano are neighbors (~ 20 km apart) on the volcanic front in Kyushu. Both the volcanoes have repeatedly erupted silicic magmas; however, their petrological features and volumes are different. Kuju volcano discharged VEI 5-6 crystal-rich silicic magma, while Aso volcano discharged VEI 6-8 crystal-poor silicic magma. Additionally, in both of the volcanoes, some amount of mafic magmas erupted with the silicic magmas, although their occurrence in the volcanic products is different. To understand differences in silicic magma systems beneath Kuju and Aso volcanoes, we focused on detailed petrological features of the silicic magmas.
We investigated ejecta of three pumiceous pyroclastic flow eruptions in Kuju volcano, which are Miyagi eruption (150 ka, 1.0 DRE km3), Simosakata eruption (120 ka, 0.2 DRE km3), and Handa eruption (54 ka, 7.2 DRE km3). In Aso’s study, we investigated ejecta of the last three large pumiceous pyroclastic flow eruptions, which are Aso-2 eruption (141 ka, 32 DRE km3), Aso-3 eruption (130 ka, 96 DRE km3), and Aso-4 eruption (90 ka, 384 DRE km3). We described petrography of the pumices and analyzed chemical composition of their whole rock, groundmass, and minerals using XRF and SEM-EDS.
The pumices of Kuju volcano mostly have a high crystal content (40 vol.%) and occasionally include andesitic mafic enclaves. Whole-rock and groundmass compositions of the pumices are 65 wt.% SiO2 and 78 wt.% SiO2, respectively. Fe-Ti oxides thermo-barometer (Andersen and Lindsley, 1988) indicated that redox state and temperature are ΔNNO+2 and 790-820 °C, respectively. Amphibole hygro-barometer (Ridolfi, 2021) shows melt water content of 5-6 wt.% and 130 MPa. On the other hand, Aso volcano ejected pumices with low crystal content (<10 vol.%), followed by mafic components as andesitic scoriae. Whole-rock and groundmass compositions of the pumices were 65-70 and 67-71 wt.% SiO2, respectively. Fe-Ti oxides in the pumices indicated that redox state and temperature were ΔNNO+1-1.5 and 790-820 °C, respectively. Amphibole in the pumices showed melt water content of 3 wt.% and 250 MPa, respectively.
The above petrological features of the ejecta show differences in the shallow magma system between Kuju and Aso volcanoes. Kuju’s silicic magmas are more oxidized, colder, and more hydrous at shallower depths than Aso’s one. Kuju volcano had the pre-eruptive reservoirs largely with crystal-rich silicic magmas close to crystal mush and partly with small patches of the mafic magma corresponding to the mafic enclaves, while the Aso volcano had layered magma reservoirs with melt-dominant silicic magmas overlying the mafic magmas corresponding to the scoriae.
The differences in occurrences of the mafic magmas in Kuju and Aso volcanoes, namely mafic magma patch in the silicic magma for Kuju and mafic magma layer underlying the silicic magma for Aso, allow us to infer that injection levels of the mafic magmas into the silicic magma bodies are determined by conditions of the silicic magmas. Thus, we calculated bulk densities of silicic and mafic magmas and investigated the injection levels based on general silicic magma reservoir model, in which silicic magma overlies thick, immobile mushy zone. In Kuju’s case, calculated bulk density of the crystal-rich silicic magma is similar to that of the mafic magma. Then, a new mafic magma probably injects directly into the crystal-rich silicic magma and becomes mafic magma patches in the silicic magma. On the other hand, in Aso’s case, calculated bulk density of the mafic magma is between those of the crystal-poor silicic magma (lighter) and underlying mushy zone (heavier). In this case, a new mafic magma likely injects between the silicic magma and the mushy zone forming a mafic magma layer.
The model of injection into silicic magma reservoir is possibly applicable to other volcanoes because the petrological features of the silicic magmas in Kuju and Aso volcanoes are common among many volcanoes in other subduction zones (e.g., Takeuchi et al., 2021). On the other hand, influence of the injection level on the variation in crystal content of silicic magma is not clear at this time; thus, further investigations are needed focusing on processes in magma reservoir such as reactivation of silicic magma by the injections and mixing of silicic and mafic magma.
We investigated ejecta of three pumiceous pyroclastic flow eruptions in Kuju volcano, which are Miyagi eruption (150 ka, 1.0 DRE km3), Simosakata eruption (120 ka, 0.2 DRE km3), and Handa eruption (54 ka, 7.2 DRE km3). In Aso’s study, we investigated ejecta of the last three large pumiceous pyroclastic flow eruptions, which are Aso-2 eruption (141 ka, 32 DRE km3), Aso-3 eruption (130 ka, 96 DRE km3), and Aso-4 eruption (90 ka, 384 DRE km3). We described petrography of the pumices and analyzed chemical composition of their whole rock, groundmass, and minerals using XRF and SEM-EDS.
The pumices of Kuju volcano mostly have a high crystal content (40 vol.%) and occasionally include andesitic mafic enclaves. Whole-rock and groundmass compositions of the pumices are 65 wt.% SiO2 and 78 wt.% SiO2, respectively. Fe-Ti oxides thermo-barometer (Andersen and Lindsley, 1988) indicated that redox state and temperature are ΔNNO+2 and 790-820 °C, respectively. Amphibole hygro-barometer (Ridolfi, 2021) shows melt water content of 5-6 wt.% and 130 MPa. On the other hand, Aso volcano ejected pumices with low crystal content (<10 vol.%), followed by mafic components as andesitic scoriae. Whole-rock and groundmass compositions of the pumices were 65-70 and 67-71 wt.% SiO2, respectively. Fe-Ti oxides in the pumices indicated that redox state and temperature were ΔNNO+1-1.5 and 790-820 °C, respectively. Amphibole in the pumices showed melt water content of 3 wt.% and 250 MPa, respectively.
The above petrological features of the ejecta show differences in the shallow magma system between Kuju and Aso volcanoes. Kuju’s silicic magmas are more oxidized, colder, and more hydrous at shallower depths than Aso’s one. Kuju volcano had the pre-eruptive reservoirs largely with crystal-rich silicic magmas close to crystal mush and partly with small patches of the mafic magma corresponding to the mafic enclaves, while the Aso volcano had layered magma reservoirs with melt-dominant silicic magmas overlying the mafic magmas corresponding to the scoriae.
The differences in occurrences of the mafic magmas in Kuju and Aso volcanoes, namely mafic magma patch in the silicic magma for Kuju and mafic magma layer underlying the silicic magma for Aso, allow us to infer that injection levels of the mafic magmas into the silicic magma bodies are determined by conditions of the silicic magmas. Thus, we calculated bulk densities of silicic and mafic magmas and investigated the injection levels based on general silicic magma reservoir model, in which silicic magma overlies thick, immobile mushy zone. In Kuju’s case, calculated bulk density of the crystal-rich silicic magma is similar to that of the mafic magma. Then, a new mafic magma probably injects directly into the crystal-rich silicic magma and becomes mafic magma patches in the silicic magma. On the other hand, in Aso’s case, calculated bulk density of the mafic magma is between those of the crystal-poor silicic magma (lighter) and underlying mushy zone (heavier). In this case, a new mafic magma likely injects between the silicic magma and the mushy zone forming a mafic magma layer.
The model of injection into silicic magma reservoir is possibly applicable to other volcanoes because the petrological features of the silicic magmas in Kuju and Aso volcanoes are common among many volcanoes in other subduction zones (e.g., Takeuchi et al., 2021). On the other hand, influence of the injection level on the variation in crystal content of silicic magma is not clear at this time; thus, further investigations are needed focusing on processes in magma reservoir such as reactivation of silicic magma by the injections and mixing of silicic and mafic magma.